This application claims the priority of Korean Patent Application No. 2002-34876 filed Jun. 21, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an image compression system, and more particularly, to an image compression method and apparatus for differentially encoding digital image signals based on a Joint Photographic Experts Group (JPEG) image compression standard.
2. Description of the Related Art
A variety of compression and decompression techniques have been suggested in order to increase transmission efficiency and to easily record/reproduce and process digital image signals when storing the digital image signals in a recording medium or transmitting the digital image signals using a communication cable.
Representative techniques for compressing and decompressing image signals include Motion Picture Experts Group (MPEG) techniques related to compression and decompression of motion pictures and JPEG techniques widely used for compressing and decompressing stationary pictures.
A JPEG standard is used to compress stationary pictures and transmit user images in image telephones using a public switched telephone network (PSTN) in which transmission bandwidth is limited and enables dynamic telecommunication by sequentially transmitting and displaying several image frames.
Also, in digital video recorders (DVRs), a compression method using JPEG compression is employed. Although the compression method using JPEG, which compresses stationary pictures, is simple and efficient, greater improvements in data compression ratio is required to store and sequentially display several frames and to transmit the frames because the compression is for stationary pictures.
FIG. 1 is a block diagram illustrating an image compression method using general JPEG compression. Referring to FIG. 1, a color mode converter 110 converts RGB image data to luminance (Y) and chrominance (Cb, Cr) data. A down sampler 120 leaves luminance components as they are and down samples only chrominance components (Cb, Cr). A discrete cosine transformer (DCT) 130 discrete-cosine-transforms the down sampled image data. A quantizer 140 quantizes the discrete-cosine-transformed image data (DCT coefficients). A Huffman coder 150 variable-length-codes the quantized image data.
Accordingly, stationary images are compressed using such a JPEG format. Here, a predetermined background color exists in a natural image. For example, a background color in an image in which a skier glides down a mountain covered with snow is white. Also, the entire background color in an image showing a sea landscape is blue. However, since an existing JPEG encoding algorithm performs discrete cosine transformation (DCT) of all pixel values representing images, the existing JPEG encoding algorithm is computationally complex, which results in a long encoding time.